Drilling for oil and gas produces large quantities of drill cuttings and drilling fluids that are comprised of mixtures of finely divided solids (silt, clay, pulverized formation minerals, barite, etc.) that are often saturated with, or suspended in, salt brine and/or organic liquid. These drilling residues require expensive disposal to protect human health and the environment.
When deposited on the landscape or simply buried, drilling residues pose unacceptable environmental hazards due to leaching of salt and organic species by water from natural precipitation that leads to ground- and surface water contamination. Attempts to stabilize drilling residue by addition of fly-ash, cement kiln dust, or other solidifying agents before burial, also have proven ineffectual and unsatisfactory largely because the salts and organic species remain leachable. Consequently, the current primary means of disposal of drilling residues is solidification, transport, and disposal in a hazardous waste landfill.
Solidification of oily drilling residues is most often accomplished by centrifugation or thermal desorption to remove the organic phase. Residual solids from centrifugation contain both leachable organic and inorganic (salt) species. Residual solids from thermal desorption also contain leachable salts. These solidified residues cannot be land-disposed on-site and must, instead, be transported and disposed at a hazardous waste landfill incurring substantial additional costs.
Solidification of drilling residues comprised of brine muds and cuttings is most often accomplished by addition of drying agents such as fly-ash or cement kiln dust. These means of solidification of drilling residue increase both the mass and volume of material that must be transported and disposed in a landfill, which increases both transportation and disposal costs.
In order to avoid the expense and liabilities associated with transportation and landfilling of hazardous drilling residues, it is desirable to render said residues suitable for land-spreading or other means of on-site disposal. On-site disposal of drilling residues requires means to separate soluble or leachable salts from finely divided dry solids or finely divided solids present in the form of paste, sludge, or slurry.
Those skilled in the art will recognize that the greatest impediment to efficient leaching or washing of fine solids is caused by the amount of interstitial liquor present in saturated fine sediments and the difficulty of separating said interstitial liquor from the surrounding fine solids.
Settled, saturated fine solids derived from drilling with salt brine muds exhibit void fractions in the range from about 40% to 55% yielding mass fractions of interstitial liquid in the range from about 20% to 35% depending on solids particulate density and concentration of salt in the interstitial liquor. If the salt concentration in the interstitial liquor is near saturation, say 24% by mass, then batch-wise washing or dilution with fresh water implies consumption of very large quantities of salt-free water in order to render saturated solids containing acceptably low concentrations (<1,000 mg/kg) of salt as required for on-site disposal.
For example, if an aliquot of saturated cuttings exhibiting a settled void volume of 40% is washed with a void volume of salt free water, the settled residue after washing will exhibit interstitial fluid having one-half the initial concentration of salt. Each successive washing stage will reduce the concentration of salt in the interstitial fluid by half again. Therefore, it would require 7 stages of washing, settling and decanting to reduce the concentration of salt from saturation (240,000 mg/L) to below 2,000 mg/L in the final interstitial fluid. Total wash water consumption would be 2.8 volumes per volume of saturated solids treated.
For on-site treatment, where fresh water needs to be transported to the site, using minimum wash water is an economic and logistical imperative. Analysis shows that the total amount of wash water needed to achieve a specified degree of salt removal is inversely proportional to the number of washing stages used. Furthermore, the minimum mass ratio of wash water to solids treated for a specified degree of salt removal is achieved by using a true counter-current, multistage, contacting regime. An additional advantage of using minimum wash water is that lower volumes of higher-concentration pregnant liquor are produced. Since high concentration dense brine has value in the oil field, the counter-current cuttings leaching system has the potential to greatly reduce disposal costs, or produce a merchantable product from drilling residues.
Disclosed herein are methods and apparatus for accomplishing true counter-current, multistage, washing and leaching, to remove soluble salts from finely divided dry solids, or finely divided solids present in the form of paste, sludge, or slurry, while producing a low volume of high concentration pregnant liquor for re-use or sale.
Those skilled in the art will recognize that disclosed methods and apparatus may be applied to other industrial separations, including but not limited to, decontamination of soils, mineral and ore beneficiation, recovery of fertilizer values from cement kiln dust, recovery of chemical values from waste solids, regeneration of ion exchange resins and adsorption media, and performing numerous liquid/solid and liquid/liquid extractions.